The performance of an optical system is usually optimal for rays travelling along an optical axis of the system. Therefore, the assembly of an optical system often includes careful alignment of the components to make the radiation travel as parallel and/or as close to the optical axis as the circumstances admit. Proper alignment is generally desirable in optical systems for charged particles as well, e.g., in electron-optical equipment.
The electron beam in a high-brilliance X-ray source of the electron-impact type is required to possess a very high brilliance. It is typically required that the electron beam spot be positionable with high spatial accuracy. As one example, the applicant's co-pending application, published as WO 2010/112048, discloses an electron-impact X-ray source in which the electron target is a liquid metal jet. The electron beam which is to impinge on the jet typically has a power of about 200 W and a focus diameter of the order of 20 μm. If the electron gun includes consumption parts, such as a high current density cathode with a limited life span, then the X-ray source may need to be disassembled regularly to allow these parts to be replaced. The subsequent reassembly may have to be followed by a fresh alignment procedure, at considerable work and/or standstill costs. A need for realignment may also arise if the X-ray source is moved physically, is subject to external shocks or maintenance.